The invention relates to compositions, methods, and apparatuses for improving the dewatering of mineral slurries. In particular, it relates to compositions and application of dewatering aids for use in the Bayer process with the aim of reducing moisture in filtered and washed alumina trihydrate.
In the typical Bayer process for the production of alumina trihydrate, bauxite ore is pulverized, slurried with caustic solution, and then digested at elevated temperatures and pressures. The caustic solution dissolves oxides of aluminum, forming an aqueous sodium aluminate solution. The caustic-insoluble constituents of bauxite ore are then separated from the aqueous phase containing the dissolved sodium aluminate. Solid alumina trihydrate product is precipitated out of the solution and collected as product.
As described at least in part, among other places, in U.S. Pat. No. 6,814,873, the Bayer process is constantly evolving and the specific techniques employed in industry for the various steps of the process not only vary from plant to plant, but also are often held as trade secrets. As a more detailed, but not comprehensive, example of a Bayer process, the pulverized bauxite ore may be fed to a slurry mixer where an aqueous slurry is prepared. The slurry makeup solution is typically spent liquor (described below) and added caustic solution. This bauxite ore slurry is then passed through a digestion stage where in a digester or a series of digesters the available alumina is released from the ore as caustic-soluble sodium aluminate. The digested slurry is then cooled, for instance to about 220° F., employing a series of flash tanks wherein heat and condensate are recovered. The aluminate liquor leaving the flashing operation contains insoluble solids, which solids consist of the insoluble residue that remains after, or are precipitated during, digestion. The coarser solid particles may be removed from the aluminate liquor with a “sand trap”, cyclone or other means. The finer solid particles may be separated from the liquor first by settling and then by filtration, if necessary.
The clarified sodium aluminate liquor is then further cooled and seeded with alumina trihydrate crystals to induce precipitation of alumina in the form of alumina trihydrate, Al(OH)3. The alumina trihydrate particles or crystals are then classified into various size fractions and separated from the caustic liquor. The remaining liquid phase, the spent liquor, is returned to the initial digestion step and employed as a digestant after reconstitution with caustic.
Within the overall process, one of the key steps is that of precipitation of the alumina trihydrate from the clarified sodium aluminate liquor. After the insoluble solids are removed to give the clarified sodium aluminate liquor, also referred to as “green liquor”, it is generally charged to a suitable precipitation tank, or series of precipitation tanks, and seeded with recirculated fine alumina trihydrate crystals. In the precipitation tank(s) it is cooled under agitation to induce the precipitation of alumina from solution as alumina trihydrate. The fine alumina trihydrate particles are re-used within the process and act as seed crystals which provide nucleation sites and agglomerate together and grow as part of the precipitation process. Larger sized crystals resulting from the precipitation step are separated and used directly as trihydrate product or more often further processed by a calcination step that produces alumina Al2O3 which is sold as product.
A key step in the Bayer process is the separation of the trihydrate crystals into the various size fractions (seed and product size material) and the subsequent separation of the crystals from the liquor from which they precipitated. The removal of the liquor is typically achieved by a filtration and water washing step. Such a filtration and washing process can be applied to both the seed crystals and/or the product sized alumina trihydrate but is most often and critically applied to the product sized material.
It is critical that the product sized alumina trihydrate has only a minimal amount of caustic liquor remaining with the filtered cake since minimal soda content in the calcined product is required to maximize the value of the resulting alumina. As a result, product sized alumina trihydrate crystals typically undergo a series of water washing steps to remove caustic. However, moisture remaining with the filtered cake can also create an issue for producers since the subsequent heating and calcination of the trihydrate requires more energy input when an increased moisture content of the filtered trihydrate product occurs. As a consequence, alumina producers aim to minimize the moisture content of alumina trihydrate product. Dewatering aids are widely used by the alumina industry to improve filtration efficiency by reducing trihydrate moistures. Such products are typically, but not exclusively, added to the water washing step in order to improve the drainage and removal of the wash water from the solids so as to minimize the residual moisture content of the filtered cake. Significant savings can be achieved in the plant by improved dewatering and deliquoring of hydrate, such savings being achieved as a result of reduced moisture levels and lower energy costs in calcination.
A wide range of variables affect the dewatering of the trihydrate. These include slurry temperature, filtrate surface tension, solid/liquid contact angle, pressure gradient across the cake and particle size distribution and shape.
As described at least in part, among other places, in U.S. Pat. No. 5,011,612, a range of dewatering aid products based on a variety of surfactant chemistries are either available or known to those familiar with the art. Such products include surfactants and/or surfactant blends containing alkylsulfosuccinates, alkyl aryl sulfonates, ethoxylated alcohols or fatty acids. Examples are mentioned in Chinese Patent application CN 200910243379 and Scientific Paper The Use of Surfactant Mixtures in the Dewatering of Alumina Trihydrate, by D. J. Fox et al, School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Australia (1987) pp. 159-163. Others include the use of fatty acid of at least 12 carbon atoms in admixture with non-ionic surfactants in U.S. Pat. No. 5,167,831. While a number of these are effective, issues remain for the industry in terms of cost and/or adverse impacts of such materials on downstream processing steps within Bayer plants. Thus there is clear need and utility for a method of improving the range and performance of dewatering aids that can be used in the Bayer process.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.